1. Field of the Invention
The present invention relates to a reflection type liquid crystal apparatus using a twistless nematic liquid crystal.
2. Description of the Related Arts
Liquid crystal apparatuses are now used in a variety of displays, as liquid crystal displays can be made smaller in size and weight compared with CRT displays. Also, liquid crystal displays give a high degree of freedom of installation, and are easily adapted to a big screen system; in particular, a liquid crystal display is expected to be successful when applied to a high-definition television.
From the viewpoint of a propagation of light in the liquid crystal, two types of liquid crystal apparatuses are employed, i.e., a transmission type and a reflection type. In an application of the liquid crystal apparatus, for example, to a high-definition television, there is a need to increase the number of pictures (pixels), and thus an area of each of the pictures must become smaller due to the increase of the number of pictures. Also, when an active matrix driver including switching transistors is used for driving the liquid crystal, a ratio of an area of each switching transistor (for example, an FET) to an area of each picture becomes large (with the reduction of an area of each picture). Also, the characteristic property of FETs is changed if light is made incident thereon, and thus preferably the liquid crystal apparatus is arranged so that light is not incident on the FETs, i.e., it becomes necessary to block any incidence of light on the FETs, and accordingly, an aperture ratio of the transmission type liquid crystal apparatus must be reduced. In the case of the reflection type liquid crystal apparatus having a reflection layer, FETs can be arranged behind the reflection layers, and thus it is possible to provide a large aperture ratio. In this regard, it can be said that reflection type liquid crystal apparatus may be adapted to displays having a large size and a high resolution.
Conventionally, the reflection type liquid crystal apparatus used for displays employs a twisted nematic liquid crystal having a positive birefringence; such a conventional reflection type liquid crystal apparatus is shown in FIG. 18 of the attached drawings. As shown in FIG. 18, the reflection type liquid crystal apparatus comprises a beam splitter 1, a twisted nematic liquid crystal 2 accommodated between opposed plates 3a and 3b, a reflecting layer 3 provided on the rear plate 3b, and a projection lens 4 for projecting an image of light produced by this reflection type liquid crystal apparatus onto a screen (not shown) beyond the projection lens 4. The twisted nematic liquid crystal 2 consists of liquid crystal molecules having major axes extending in parallel to the plates 3a and 3b, and the axes are oriented rotatingly (twistingly) along the thickness of the liquid crystal from a position nearer to one of the plates 3a and 3b than to the other plates.
The beam splitter 1 splits light from a source (not shown) into a polarized light S and a polarized light P. Namely, the polarized light S having a plane of vibration in parallel to an incident plane to the beam splitter 1 is reflected by the beam splitter 1, and the polarized light P having a plane of vibration normal to the incident plane is transmitted by the beam splitter 1. Therefore, only the polarized light S is incident on the liquid crystal 2, reflected by the reflecting layer 3, and emitted from the liquid crystal 2. The plane of vibration of the polarized light rotates in accordance with one of several predetermined modes, while the polarized light propagates through the liquid crystal 2, so that in one circumstance, the emerging polarized light has the same plane of vibration as that of the incident polarized light S, and in another circumstance, the emerging polarized light has a different plane of vibration from that of the incident polarized light S. When the emerging polarized light has the same plane of vibration as that of the incident polarized light S, it is again reflected by the beam splitter 1 and does not reach the projection lens 4, and thus a black spot is produced on the screen located beyond the projection lens 4. When the emerging polarized light P has a different plane of vibration from that of the incident polarized light S it is transmitted by the beam splitter 1 and reaches the projection lens 4, and thus a white spot is produced on the screen.
In this conventional reflection type liquid crystal apparatus, the arrangement is such that, when a voltage is not applied to the liquid crystal 2, the emerging polarized light has the same plane of vibration as that of the incident polarized light S, and thus does not reach the projection lens 4 to thereby form a black spot on the screen. This is based on the fundamental nature of the twisted nematic liquid crystal, known as an optical activity. Namely, the polarized light S incident on the liquid crystal 2 is propagated in the latter with its plane of vibration rotated in conformity with the twist of the major axes of the molecules of the liquid crystal 2 (optical activity mode), is reflected by the reflecting layer 3, and is returned with its plane of vibration rotated in reverse, thereby to emerge from the liquid crystal 2. Accordingly, the emerging polarized light has the same plane of vibration as that of the incident polarized light S since the planes of vibration in the forward and return propagations are rotated by the same amount but in the opposite directions. Thus this emerging polarized light is reflected by the beam splitter 1 and is not transmitted to the projection lens 4, to thus cause a black spot on the screen. In this liquid crystal apparatus in which the propagation of the light is carried out in the optical activity mode, the arrangement is such that the plane of the vibration of the incident polarized light S coincides with the orientation of the axes (director) of the molecules of the liquid crystal 2 located near the front plate 3a, and accordingly, the incident polarized light S is not affected by a double refraction of the liquid crystal 2.
Then, when the voltage is applied to the liquid crystal 2, the molecules of the liquid crystal 2 tilt up relative to the plates 3a and 3b. If the molecules of the liquid crystal 2 are fully raised, however, the incident polarized light S is propagated along the major axes of the molecules of the liquid crystal 2 and either the optical activity or the double refraction does not occur, and thus the plane of vibration does not substantially vary, with the result that the emerging polarized light is the same as the incident polarized light S. This emerging polarized light S will be reflected by the beam splitter 1, the black spot maintained on the screen, and the light image not produced.
To produce the light image, the plane of vibration of the emerging polarized light must be changed from the plane of vibration of the incident polarized light S. To this end, it is necessary to regulate the applied voltage so that the molecules of the liquid crystal 2 are partially raised. In this state, a portion of the molecules of the liquid crystal 2 which are located near the plates 3a and 3b tends to maintain its original position extending in parallel to the plates 3a and 3b, and portions of the molecules of the liquid crystal 2 which are located inside of the plates 3a and 3b are gradually raised in accordance with their locations along the thickness of the liquid crystal 2. As a result, the propagation of the incident polarized light S in the liquid crystal 2 is not carried out in the optical activity mode but in the double refraction mode. In the double refraction mode, there is a difference in the velocity of ordinary light and that of extraordinary light, and the resultant polarized light rotates. As a result, the emerging polarized light has a different plane of vibration from that of the incident polarized light, and thus can be transmitted by the beam splitter 1 to thereby cause a white spot on the screen.
This conventional reflection type liquid crystal apparatus suffers from a problem in that the design of the liquid crystal panel is difficult because, in order to make a white (or a colored spot) spot on the screen, it is necessary to change the propagation of the liquid crystal 2 from the optical activity mode to the double refraction mode by applying a voltage to the liquid crystal 2, and in this double refraction mode, the polarized light must be propagated in the double refraction mode in the liquid crystal 2 which still maintains a twist structure. In addition, it is necessary but difficult to determine an optimum voltage to be applied to the liquid crystal 2 because, as described above, the double refraction does not occur if the molecules of the liquid crystal 2 are fully raised, and the voltage must be controlled so that the molecules of the liquid crystal 2 are partially raised so that the emerging polarized light has a particular plane of vibration different from that of the incident polarized light S. Also, even if an optimum voltage is determined, the state of the liquid crystal 2 itself is apt to fluctuate. Accordingly, a problem arises in that the contrast of the image is unstable. Also, it is necessary to consider an influence of the wavelength of the source of light. If the source has a wide band width wavelength, the plane of vibration of the obtainable polarized light varies in accordance with the wavelength of the source, leading to a variation of the brightness. Therefore, it is necessary to take measures in this regard, and when using the reflection type liquid crystal apparatus with the twist nematic liquid crystal, it may be necessary to adopt a complex arrangement, for example, by providing an additional compensating liquid crystal panel having an opposite twisting characteristic in addition to a main liquid crystal panel having a particular twisting characteristic.